1. Field of the Invention
The present invention relates to a ceramic heater suitable for a ceramic glow plug disposed in diesel engines, as well as to the ceramic glow plug. The present invention also relates to a method of manufacturing such a ceramic heater.
2. Description of the Related Art
In manufacture of a conventional silicon-nitride ceramic heater including a heating element embedded in a ceramic that contains silicon nitride as a main component, Al.sub.2 O.sub.3 --Y.sub.2 O.sub.3 and oxides of rare earth elements have been used as sintering aids (Japanese Patent Application Laid-Open (kokai) Nos. 5-1817, 5-174948, 5-234665, and others).
The present inventors experimentally manufactured the above-mentioned conventional ceramic heaters and tested them to discover the following disadvantages:
A silicon-nitride ceramic manufactured through use of Al.sub.2 O.sub.3 --Y.sub.2 O.sub.3 as a sintering aid has poor high-temperature strength and acid-resistance.
A silicon-nitride ceramic heater manufactured through use of an oxide of a rare earth element as a sintering aid is superior to the silicon-nitride ceramic heater manufactured through use of Al.sub.2 O.sub.3 --Y.sub.2 O.sub.3 as a sintering aid in terms of both high-temperature strength and acid resistance. However, the acid resistance of the silicon-nitride ceramic heater manufactured through use of an oxide of a rare earth element is insufficient in the case where the temperature of the ceramic heater is increased to a temperature as high as 1400.degree. C. in order to improve ease of starting an engine.